In a heatproof bowl set over a saucepan of barely simmering water melt the chocolate and let it cool to room temperature. In a saucepan stir together the sugar and the water and cook the mixture, stirring, until the sugar is dissolved. In a bowl with an electric mixer beat the egg yolks until they are thick and pale. Bring the sugar syrup to . boil over high heat and beat it into the yolks. Pour the custard into the saucepan and cook it over moderately low heat. stir ring, until it is thickened and a candy thermometer registers 160° F. Return the custard to the bowl. cleaned, and with the electric mixer, with cleaned beaters, beat the melted chocolate and the cinnamon into it. Let the mixture cool to room temperature. In another bowl beat the cream until it holds soft peaks, fold the whipped cream into the chocolate mixture. and chill the mixture, covered with plastic wrap, for 1 hour. Freeze the mixture in an ice cream freezer according to the manufacturer .s instructions. Makes about 1 quart.